Yoga courses are run by various centers around the world to train both yoga practitioners and yoga teachers the basics and advanced techniques of yoga.
If you're looking for yoga courses to improve your teaching skills you should do a lot of research into the center you are considering - try to find reviews, talk to people who took the yoga course and find out what they thought of it and if they thought it was good value for money.
If you are looking for yoga courses in order to learn yoga then you should consider booking a private yoga course with a private yoga teacher first. Some yoga courses that are run in schools require at least a basic knowledge of yoga and a level of flexibility and suppleness to hold the various poses. A private yoga teacher will be able to work with you to increase your skills and abilities to get you up to a good enough standard to take yoga courses.